The present invention relates to a thin film lithium niobate structure and, more particularly, to using an ion implanted etch stop to form a film of virtually any desired thickness with excellent uniformity.
Lithium niobate (LiNbO3) and other ferroelectric materials are often used as waveguiding layers in various optical devices such as, for example, optical switches, electro-optic modulators and the like. In these applications, it is particularly advantageous to be able to form relatively thin (i.e.,  less than 1 xcexcm) layers of such films, due to their large optical confinement properties and strong optical nonlinearities.
Many techniques have been used in the past to form thin ferroelectric oxide films. In most cases, for example with LiNbO3, a liquid phase epitaxy (LPE) process is used. In an exemplary LPE method, Li2Oxe2x80x94V2O5 is used as an LPE growing flux, and the raw materials are weighed and mixed in such a way that the melt composition becomes LiNbO3:Li0.7Na0.3VO3=20:80 (mol %), and the mixture, placed in a platinum crucible, is set in a furnace. The mixture is melted at 1000xc2x0 to 1100xc2x0 C. to have an even composition, and is then over-cooled to or below a saturating temperature. Next, a suitable substrate (such as LiTaO3), attached to a platinum substrate holder with the +z face of the substrate facing downward, is inserted in the furnace and is sufficiently preheated on the flux. The resultant structure is then isothermally grown by, for example, a one-side dipping system. In one conventional arrangement, the growing temperature is between 930xc2x0 and 950xc2x0 C., the number of rotations of the substrate at the time of growth is 10 to 100 rpm, and the growing speed is about 1.0 xcexcm per minute.
Although this method is suitable for forming waveguiding structures in lithium niobate, the quality of the material in the waveguide-formed region is often less than desirable. In a best case, it would be preferred to fabricate a thin film of lithium niobate from a bulk grown LiNbO3 substrate, since bulk grown LiNbO3 is of much higher quality than materials previously used, thus improving the quality of the grown film.
Thus, a need remains in the art for a method of providing high quality, thin film lithium niobate structures.
The need remaining in the prior art is addressed by the present invention, which relates to a thin film lithium niobate structure and, more particularly, to using an ion implanted etch stop to form a film of virtually any desired thickness (e.g., xe2x89xa615 xcexcm).
In accordance with the present invention, a single crystal bulk lithium niobate substrate is subjected to ion bombardment so as to create a xe2x80x9cdamagedxe2x80x9d layer at a predetermined distance below the substrate surface. The implant energy determines the depth of this damaged layer below the surface. Following the ion implant, a heat treatment process is performed, where the heat treatment serves the dual purpose of xe2x80x9chealingxe2x80x9d some of the damage in the lithium niobate material between the damaged layer and the surface, and modifies the etch rate of the damaged layer. Hereinafter, the xe2x80x9cdamaged layerxe2x80x9d will be referred to as the xe2x80x9cetch stop layerxe2x80x9d. Subsequent to the heat treatment process, the etch properties of the lithium niobate bulk material and etch stop layer are sufficiently different that a conventional wet chemical etch may be used to form the desired thin film lithium niobate structure.
In one embodiment of the present invention, the ion implant process is performed to yield an etch stop layer at a relatively shallow depth (e.g., 2 xcexcm) below the substrate surface. Subsequent to the heattreatment step, the substrate is bonded to a xe2x80x9chandlexe2x80x9d wafer, where the substrate surface that had been subjected to the ion bombardment is bonded to the handle wafer (i.e., the substrate is turned xe2x80x9cupside downxe2x80x9d and bonded to the handle wafer). The exposed bulk of the lithium niobate substrate is then removed by a conventional wet chemical etch and will stop, in accordance with the present invention, at the etch stop layer created by ion bombardment. The remaining lithium niobate material, therefore, will be the relatively thin, 2 xcexcm top surface region of the original substrate. Therefore, in accordance with the present invention, a thin lithium niobate film is formed from the original bulk substrate material.
In another embodiment, a ridge waveguide structure (or any other patterned structure) may be formed by first masking the surface of the lithium niobate bulk crystal substrate prior to the ion implantation. As before, a heat treatment process is used to modify the etch rate characteristics of the ion implanted regions with respect to the remaining substrate material. A following wet chemical etch will then preferentially remove the original lithium niobate substrate material with respect to the ion bombarded layer.
Various and other embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.